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From: halbert@crl.dec.com (Dan Halbert)
Newsgroups: comp.object
Subject: Re: Examples of Multiple Inheritance?
Message-ID: <1990Dec6.185634.24199@crl.dec.com>
Date: 6 Dec 90 18:56:34 GMT
References: <60700005@inmet> <1990Dec6.020928.13319@Think.COM>
Sender: news@crl.dec.com (USENET News System)
Reply-To: halbert@crl.dec.com
Organization: DEC Cambridge Research Lab
Lines: 32

[reposted with wrapped lines - sorry]

The DEC Trellis language provides multiple inheritance. We coded the
programming environment for Trellis in Trellis, and found multiple
inheritance to be very useful. However, most uses are not of the
Teacher/Student type of combination, where two conceptually "equal"
types are merged. Instead, we usually use multiple inheritance to
guarantee that a certain type will obey some protocol, ancillary in
nature.

For example, a number of the basic data types provided in the Trellis
type library are subtypes of the abstract types Readable and/or
Printable, which define operations for reading and writing
representations of these types to and from streams. Another common
ancillary abstract type is Hashed, which defines an operation that
returns an Integer hash value for an object. Integer, for instance is a
subtype of both Readable and Hashed (Readable is a subtype of
Printable).

In our window system code, we also used multiple inheritance.
We call widget-like objects "frames". Some frames, in addition to
their mainline inheritance, are subtypes of type Selector, which means they
display things which can be selected (e.g. text or lists).

These ancillary abstract types define operations, but often don't
provide implementations for them, or provide a general purpose
implementation. If there is no implementation, it is up to the subtypes
that inherit them to provide implementations. The Trellis compiler
checks that non-abstract types have implementations for all defined
operations.

--Dan Halbert